The present invention relates to the field of machine recognition of magnetically imprinted characters on a document and more particularly to a method and apparatus for generating a variable threshold which is applied to a digitized waveform representing an unknown character for deriving peak values which are processed for use in identifying the unknown characters.
In single-gap magnetic character reading systems, a single analog input waveform is obtained by passing the characters to be sensed, normally printed on a document, beneath a magnetic read head at least as wide as the height of the characters and having a single flux gap. The signal generated by the read head is a derivative waveform representing the rate of change of magnetic flux transversing the head as the characters are scanned. Since the distribution of ink, and thus flux, associated with each different character is unique, the waveform derived for each different character uniquely identifies that character.
To simplify the timing of the waveform analysis process, the characters are provided with stylized geometric features which impart anticipatable timing characteristics to the derived waveforms. Thus, in accordance with this means for reader identification, each character of the E-13B font is divided into a predetermined number of vertical segments corresponding to a location of bar portions of the character. The characters are designed such that the distribution of ink undergoes significant change only at the boundaries between each segment. Hence, peak fluctuations in the derived waveform caused by these variations in ink distribution can occur at only predetermined times during the character scan.
Prior character readers have utilized single or multiple threshold values to eliminate noise levels due to ink splatter, smudges or smearing of the ink which occur during the printing of the documents, since recognition of the unknown characters is based on utilizing the amplitude of each of the peaks of the generated waveform for correlation with known peak amplitude characteristics of each of the E-13B characters to identify the character read. An example of this type of recognition system may be found in U.S. Pat. No. 3,534,334. Usually this type of threshold level is established on the conservative side to insure that all types of noise peaks will be eliminated. Since the exact location of each peak in the waveform is critical to the recognition of the unknown character, it is imperative that the start of the analog waveform be determined as accurately as possible since any variation in the location of the peaks in the waveform will shift the location of the peaks from their true position, thereby reducing the accuracy of the recognition operation. It is therefore an object of this invention to provide a magnetic character recognition system which provides highly accurate data in response to the generation of an analog signal representing an unknown character. It is a further object of this invention to provide a character recognition system which detects accurately the start of the character waveform. It is another object of this invention to provide a character recognition system which minimizes the effects of variations in ink intensity found in the characters that are to be read. It is still another object of this invention to provide a character start circuit which will detect the actual start of a character at a high relative speed, yet which circuit is low in cost.